1. Field of the Invention
The present invention relates to a driving apparatus for controlling the frequency of a laser beam output from a semiconductor laser, and more particularly to a driving apparatus to generate frequency modulated waves using the semiconductor laser.
2. Description of the Related Art
Before now, it has been difficult to attain frequency modulated waves using a laser beam generated by a semiconductor laser. This is because of the variation among semiconductor laser elements in their current-oscillation frequency characteristics and also because of the great temperature dependence of these current-oscillation frequency characteristics.
For this reason, modulation systems for phase modulation and so forth, specifically modulation systems excluding frequency modulation, have been employed up to now as modulation systems in optical communications, and so forth.
FIG. 5 is a graph showing roughly the current-oscillation frequency characteristics of semiconductor lasers. In FIG. 5, the horizontal axis shows the current supplied to the semiconductor laser and the vertical axis shows the oscillation frequency of the semiconductor laser.
As understood from FIG. 5, the oscillation frequency of the laser beam decreases as the current supplied to the semiconductor laser increases. Here, as shown in FIG. 5, this oscillation frequency deviates in a roughly linear manner in relation to the supplied current, but it shifts abruptly at certain specific current values i.sub.1, i.sub.2, i.sub.3, . . .
The current value supplied to the semiconductor laser may be changed in order to effect frequency modulation of a laser beam output from this semiconductor laser.
Here, in order to attain a good frequency modulated wave, the oscillation frequency must be changed only in the regions where the current dependence of the oscillation frequency (relationship of the current value variation .DELTA.i and the oscillation frequency variation .DELTA.f) becomes linear. Consequently, the current value supplied to the semiconductor laser must be varied only in the regions of A.sub.1, A.sub.2, . . . in FIG. 5.
On the other hand, when the current values i.sub.1, i.sub.2, i.sub.3, . . . are included in the variable range of the current value (see symbols B.sub.1, B.sub.2, B.sub.3, . . . in FIG. 5), the oscillation frequency shifts abruptly at these current values i.sub.1, i.sub.2, i.sub.3, . . . ; therefore, the relationship of the current value variation .DELTA.i and the oscillation frequency variation .DELTA.f does not become linear. Consequently, demodulation of the modulated wave attained in this way becomes very difficult. For this reason, in the case where the laser beam output from the semiconductor laser is to be frequency modulated, the current values included in the ranges B.sub.1, B.sub.2, B.sub.3, . . . in FIG. 5 (specifically current values i.sub.1, i.sub.2, i.sub.3, . . . and current values in the vicinity thereof) must not be used.
The current values i.sub.1, i.sub.2, i.sub.3, . . . , where the oscillation frequency shifts abruptly, differ among the semiconductor laser elements. Also these current values i.sub.1, i.sub.2, i.sub.3, . . . vary greatly depending on temperature.
FIG. 6 is a graph showing roughly the temperature dependence of the current-oscillation frequency characteristics of semiconductor lasers. As shown in FIG. 6, the current values i.sub.1, i.sub.2, i.sub.3, . . . , where the oscillation frequency shifts abruptly, shift to the right (specifically, the direction in which the current values i.sub.1, i.sub.2, i.sub.3, . . . rise) as the temperature of the semiconductor laser elements rises.
For the reasons explained above, in the case of frequency modulation of a laser beam output from a semiconductor laser, the ranges of current used must be determined in consideration of both the current-oscillation frequency characteristics of each semiconductor laser element and the temperature dependence of these current-oscillation frequency characteristics.
For this reason, driving control becomes very complex and large scale, thereby becoming an obstacle to realizing frequency modulation using semiconductor lasers.